My staffroom love's gone sour
A: No, not yet. I won't lecture you about keeping your professional and personal lives separate - because many good relationships begin, and are sometimes sustained, in the workplace. But trouble can start when there's a break-up.
Anyone who threatens to make life hard for you has been unprofessional.
This could be harassment, and it's unacceptable. But is he in a position to make life tough because he's, say, your line manager? If so, that makes things much worse. He may even have abused his position. But be very careful about making any accusations. If you do that, you'll need hard evidence, witnesses who can do more than repeat hearsay, solid support and a motive that is more than a personal matter of revenge.
Senior managers will take an interest if your work, or his, or the work of the teams you work in starts to suffer. If you and he are at odds, and if your colleagues take sides, the workplace as a whole will suffer. Your professionalism requires you to be calm and detached for the sake of the effective running of the school.
Let a trusted colleague know the situation, and tell that person about the threats. Being open about this sensitive subject will help you to clarify your own views and feelings, and it will help you to get the protection of an objective observer.
Finally, keep a timed and dated log of any incidents - if your own actions don't nip this in the bud, you may need credible evidence later to support you in any further developments.